1. Field of the Invention
This invention relates generally to the fabrication of master records and direct read after write ("draw") records using an information modulated radiation write beam, and it relates more particularly to video master records and video direct read after write records. The invention also relates to records particularly adapted to the method of fabrication, but also having advantages unrelated to the method of fabrication.
2. Description of the Prior Art
Several methods have been developed in the past for writing information onto an information storage medium using an information modulated radiation write beam. The information storage medium, which may be a video master record or a video "draw" record, has a radiation sensitive layer on at least one of its faces and is exposed to the radiation write beam during writing. In the article "A Review of the MCA Disco-Vision System" (Journal of the SMPTE, July '74, Volume 83) (hereby incorporated by reference) a method, typical to the prior art, for fabricating a master video record is described. The master record comprises a glass disc base on which a thin metal film is evaporated. This film is sensitive to a radiation write beam provided by an argon-ion laser. The write beam is modulated by a Pockels cell and directed by optics onto the metal film of the rotating disc, to produce an information track consisting of small holes in the metal film where the metal film is evaporated by the beam. The last few optical elements in the write beam are mounted on a carriage that is moved along the record's radius by a motor-driven lead screw. The objective lens is supported on an air bearing, which is loaded against the surface of the record. A relatively small air flow at moderately high pressure maintains the head and objective lens at a constant distance of approximately 0.0005 inch (0.013 mm) from the metal film surface.
A master record thus fabricated may also be used as a direct read after write record. Since reproduction from a direct read after write record provides a higher quality television picture than may be provided by video magnetic tapes, it is believed that there may be a potential in the future for the use of direct read after write records for recording purposes as a high quality alternative to magnetic tape recording.
The problems presently encountered in master and "draw" record fabrication are mainly due to dust particles settling on the surface of the information layer impairing the formation and read-out of the information track. Also, with methods wherein the write beams evaporates material out of a metal film, evaporated material may settle on the objective lens, thus interfering with the optical properties of the lens.
The last mentioned problem could be overcome by using a different kind of information layer, such as a photographic or photochemical layer. At the present state of the art however, it is not possible to monitor the recording with these layers as compared to a photomechanical layer such as the already described metal film, due to the fact that the virtual image immediately available on recording must be developed before it can be formed into distinct radiation read beam modulating "pits", measuring roughly 1 .mu.m .times. 2 .mu.m at a mean distance in the track of 1.66 .mu.m.
The first mentioned problem may be overcome by carrying out the method in an ultra clean room, having special and elaborate equipment to filter most of the dust out of the air.
It is an object of the invention to provide a method that will alleviate the above mentioned problems and that may be used outside of an ultra clean room, for instance for the direct recording prior to broadcasting of television programs in or outside a television studio.
It is a further object of the invention to provide a record which is particularly adapted for use with the new method. Another object is to provide an information storage record that has information tracks provided in a surface layer and which may be reproduced without loss of quality of the reproduced information due to dust particles on the information tracks.